Sensor array and method for determining the density and viscosity of a liquid

ABSTRACT

A sensor arrangement for ascertaining the density and the viscosity of a liquid is proposed, having an arrangement comprising at least two basic sensor elements, at least one of which can be wetted with the liquid, and having electro-acoustical transducers ( 6 ) in the basic sensor elements for generating and detecting surface acoustic waves with predetermined wave modes, from whose propagation behavior along a measurement path a measure for the density and the viscosity of the liquid can be ascertained. Liquid traps ( 17 ) for the liquid, which extend in the applicable measurement path, are disposed in the region of at least one of the basic sensor elements, parallel to the direction of propagation of the surface acoustic wave.

PRIOR ART

The invention relates to a sensor arrangement for ascertaining thedensity and viscosity of liquids, and to a method for performing thisascertainment, as generically defined by the preamble to the main claim.

In general, in a density measurement the mass of a known volume ofliquid is ascertained using simple measurement arrangements. Inaddition, the resonance mistuning can be ascertained and evaluated todetermine the density in a tube, through which the examined liquidflows, in an acoustical measurement arrangement. Well-known measuringmethods that can be used to measure the viscosity of the liquid arerotation viscosimetry and trap ball viscosimetry. It is common to allthe methods named that the two measurement variables of density andviscosity must be ascertained using different apparatuses, which eachrequire a great deal of space, are cost-intensive when there is a demandfor high measurement precision, and require relatively large liquidvolumes for the measurement.

In view of an ever-increasing necessity for miniaturization and systemintegration, there is a need for compact, cost-effective apparatuses forhigh-precision on-line density and viscosity measurement in smallvolumes of liquid, but this need cannot be met with the measurementapparatuses available today. Examples of such an application aremeasuring the density and viscosity when metering diesel fuels in motorvehicles, on-line monitoring of the status of motor oils, or thedevelopment of microfluidic analysis systems in chemistry or medicine,for example for studying such physiological media as blood or urine, orfor producing pharmaceutical products.

Microsensors for density and viscosity measurement of liquids can beclassified in two categories, in accordance with their fundamentalfunctional principles. The first is so-called surface acoustic wavesensors (SAW sensors), which work by using an interaction between thepropagation path of a surface acoustic wave or a bulk wave and theliquid to be studied, and the second is sensors whose measuringtransducers comprise resonantly vibrating microstructures.

In the sensor arrangement of the generic type involved here, the pointof departure is a known measurement principle that is described forinstance in the article entitled “A study of Love-wave acousticsensors”, J. Du, G. L. Hardling, P. R. Ogilvy and M. Lake, in theprofessional journal Sensors and Actuators A56 (1996), pages 211-219.With the measurement layout described here, a sensor is realized inwhich work is done with horizontally polarized acoustic shear waves assurface waves, that is, so-called leaky waves or surface skimming bulkwaves (SSBWs), or Love waves. These acoustic wave modes are generatedand also detected with so-called interdigital transducers, known per sefrom the aforementioned prior art, so that from the propagation behavioralong a propagation or measurement path, the desired sensor signal canbe obtained.

ADVANTAGES OF THE INVENTION

The sensor arrangement of the applicable generic type recited at theoutset for ascertaining the density and the viscosity of a liquid isadvantageously refined in accordance with the invention as defined bythe characteristics of the body of the main claim and the coordinatemethod claim.

This sensor arrangement according to the invention, by utilizing theinfluence of additional interferences, imposed in a targeted way on thesensor surface of a basic sensor element, in a propagation path for theacoustic waves advantageously enables a separate measurement of densityand viscosity of a liquid in a measurement layout with high measurementprecision. In the known arrangement referred to at the outset,conversely, in a measurement using Love wave modes, it is possible onlyto detect a density-viscosity product.

Per se, a viscosity and density sensor with a so-called quartz crystalmicrobalance (QCM) for measurement with bulk waves, rather than surfacewaves, is known in which similar interferences are provided in the formof liquid traps. This is described for instance in the paper entitled“Measuring Liquid Properties with Smooth-and Textured-SurfaceResonators”, by S. J. Martin et al, IEEE 1993 International FrequencyControl Symposium, pages 603-608. Here the surface of the oneoscillator, for instance, is provided with walls of metal, such as gold,that are oriented perpendicular to the direction of oscillation. Thepockets between the walls act as liquid traps, and the liquid locatedtherein executes the oscillation motion regardless of its viscosity.

This known quartz crystal microbalance is a thickness shear oscillator,which is excited by low electrodes, utilizing the inverse piezoelectriceffect. Since in a liquid phase, because of the shear motion, no directprojection of acoustical energy occurs, because shear modes are notcapable of propagation in liquids, the QCM is suitable for studyingliquids as well. Often, a change in resonant frequency is measured bymass accumulation, and the QCM acts as a frequency-determining elementin an oscillator circuit.

The invention advantageously exploits the effect that in viscousliquids, because of viscous coupling, a frequency shift dictated by theviscosity and density of the liquid additionally occurs. This can beused to ascertain the density-viscosity product of the liquid, but inaddition the influence of density from the influence of viscosity can bedistinguished with the layout proposed according to the invention, sothat both variables can be measured independently of one another.

Thus in a refinement of the generic arrangement, at least two basicsensor elements, operated parallel in terms of their construction, areadvantageously used, and the advantages of using surface waves,especially SSB waves or Love waves, can be exploited. These advantagesare above all a high measurement sensitivity, the use of transducerelectrodes that are protected from the liquid, an inert surface, and lowcross sensitivity.

Compared to the use of the known QCMs, in the arrangement of theinvention the application of gold by electroplating can be dispensedwith, and the entire sensor arrangement can be produced in asemiconductor-compatible production process. Since the gold used in theknown arrangement with QCMs has a very high density compared to theliquid, with the layout according to the invention, whose materials arecloser in density to that of the liquid, the measurement sensitivity canalso be enhanced by comparison.

With the claimed measurement method, a measurement signal that is easyto process further can be obtained in a simple way by the evaluation offrequency shifts. The frequency shifts of the basic sensor element withthe liquid traps, in addition to the influence of the density-viscosityproduct, has a dependency that is dictated only by the density of theliquid and by the effective volume of the liquid traps. If the frequencyshifts of the two basic sensor elements are then linked together, thedensity and viscosity of the measurement liquid can be ascertainedseparately.

With the present invention, a microsensor is proposed, with which thedetermination of the density and the viscosity of volumes of liquid inthe microliter range is possible with high resolution and highmeasurement precision. This sensor can be produced economically in batchprocesses that are suitable for mass production, with recourse tomethods known from semiconductor manufacture. Thus the advantages ofsensors which generate a measurement signal by utilizing the interactionbetween the propagation path of a surface acoustic wave and the liquidto be studied, and other sensors (such as bulk mode sensors or QCMs) canthus be combined, and the various specific disadvantages thereof areavoided.

These and further characteristics of preferred refinements of theinvention are disclosed not only in the claims, including the dependentclaims, but also in the specification and drawings; the individualcharacteristics can each be realized alone or in various subsidiarycombinations in the embodiment of the invention and in other fields andcan represent versions that are advantageous and are worthy of patentprotection on their own, for which patent protection is here claimed.

DRAWING

Exemplary embodiments of the sensor arrangement of the invention will bedescribed below in conjunction with the drawing. Shown are:

FIG. 1, a schematic view of a sensor arrangement for ascertaining thedensity and the viscosity of a liquid flowing through the sensorarrangement;

FIG. 2, a detail of an interdigital transducer for generating anddetecting acoustic waves;

FIGS. 3-5, variant arrangements of the interdigital transducer of FIG.2;

FIGS. 6-8, sections through a substrate of the sensor arrangement, withdifferent versions of the coating;

FIG. 9, a more-detailed plan view of two propagation paths of themeasurement liquid in the sensor arrangement;

FIG. 10, a basic circuit diagram of an evaluation circuit coupled to thesensor arrangement; and

FIG. 11, an evaluation circuit that is expanded compared to FIG. 10.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

In FIG. 1, a sensor arrangement 1 is shown in a basic cutaway view; ameasurement fluid flows through this arrangement from an inlet 2 to anoutlet 3 in the direction of the arrow 4, so that its density andviscosity can be determined. The primary component of the proposedsensor arrangement 1 is a substrate 5, polished on one side, of apiezoelectric material in which horizontally polarized acoustic shearwaves can be excited by basic sensor elements and are capable ofpropagation. As substrate materials, Y-rotated quartz slices, somelithium niobate and lithium tantalate slices, and correspondinglypolarized piezoelectric ceramics are suitable.

Located on the polished surface of the substrate 5 is an arrangement ofmetal interdigital transducers (IDTs) 6, which will be explained infurther detail in conjunction with FIG. 2. These interdigitaltransducers 6 are for instance of aluminum, titanium, chromium, gold orplatinum, optionally on an adhesion layer of titanium or silicon, andserve to excite and detect the surface acoustic waves.

In FIG. 2, one of the interdigital transducers 6 is shown in detail;transducer prongs 7 are capable of generating acoustic waves at thewavelength 8 (medium frequency) upon excitation by an electrical voltageat an input 9. The result is a surface acoustic wave, that is, inparticular a shear wave, in the polarization direction indicated by thearrow 11, with the aperture indicated by arrow 12. In an exemplaryembodiment not shown here, the transducer prongs 7 can also be splitwithin the period into two individual prongs or split prongs, thuscreating λ/8 prongs. Between the electrical and the mechanical period isthe factor of 2 in this case, so that it is possible for internalreflections and the so-called triple transit echo (TTE) to be eliminatedor at least reduced.

The arrangement of the interdigital transducers 6 in the sensorarrangement 1 of FIG. 1 can be embodied in accordance with the exemplaryembodiments of FIGS. 3-5. For instance embodied as a delay line with atransmitting IDT 6 a, a propagation path 13 and a receiving IDT 6 b, orin FIG. 4 as a two-gate resonator or in FIG. 5 as a one-gate resonatorwith one or two IDTs 6 and with reflector banks 14.

The sensor arrangement 1 of FIG. 1 includes two basic elements, disposedparallel to one another, with the interdigital transducers 6, but forthe sake of simplified evaluation and improved temperature compensationof the measurement signals, a third parallel basic sensor element, notshown in this figure, can also be provided. Also in the exemplaryembodiment of FIG. 1, next to or in between the basic elements with theIDTs 6 on the surface of the substrate 5, there is a thin-filmtemperature resistor 15 of meandering shape, since the viscosity inparticular is highly temperature-dependent, and thus the temperaturerepresents a further important measurement variable. As the material forthe thin-film temperature resistor 15, the material as for the IDTs 6can advantageously be considered, namely titanium/platinum ortitanium/platinum/titanium, and the adhesion layer can be eithertitanium or silicon.

On the substrate 5 of FIG. 1, above the basic elements with the IDTs 6,an acoustic waveguide layer 16 is provided, which can for instancecomprise an ormocer, a silicon compound, or a polymer, so that thegeneral shear mode (leaky wave or SSBW) of the acoustic wave becomes aso-called waveguide mode (in this case, a Love wave). To distinguish theeffect of density from the effect of viscosity in the measurement,mechanical interferences in the form of liquid traps 17 areintentionally disposed above the basic element having the IDTs 6, andinside these liquid traps, because of the mechanical discontinuity, theacoustic wave is incapable of propagation.

To that end, the region before, above and between the variousinterdigital transducers 6 is provided with the walls 18, orientedparallel to the direction of propagation of the acoustic wave, and thegeometric options for the disposition of these walls will be explainedin conjunction with FIGS. 6-8 and the plan view of FIG. 9. These liquidtraps 17 can be produced here in the form of trenches 22, as shown inFIG. 6, or as pits or sponges, not shown, by suitable structuring of alayer 20 located above the interdigital transducer 6. Between the layer20 and the interdigital transducer 6, a further intermediate layer 21can be provided to improve the adhesion and/or to protect the IDTs 6. Inthe arrangement of FIG. 6, the so-called leaky waves or SSB waves areused.

If the component is a so-called Love mode component, then the liquidtraps of FIG. 7 can also be created directly in a waveguide layer 23,which otherwise corresponds to the waveguide layer 16 of FIG. 1, bymaking trenchlike etched features 22. The thickness of the waveguidelayer 23 above the second, parallel basic element having the IDTs 6without the liquid traps can thus be reduced enough that the samesensitivity of the two basic elements is achieved.

Another method, shown in FIG. 8, for creating liquid traps 17 ortrenches 22 for waves of the Love mode type is to apply and thenstructure a further liquid trap layer 25 above the acoustic waveguidelayer 23, optionally also using an additional intermediate layer 21 asan adhesion promoter and/or as an etch stop layer, similarly to theexample of FIG. 6. In this way, the replicability of the trench depth isimproved.

In all the versions, the formation of liquid traps 17 by providing pitsof circular or polygonal cross section or by providing a spongelikesurface structure, not shown here, can be done as mentioned above. Inall the cases described, a thin metal shielding layer 26, which can befrom a few nanometers to 100 nm thick, is provided above the liquidtraps in order to shield against unwanted acoustoelectrical interactionsbetween the measurement liquid and the sensor arrangement 1. In theexemplary embodiment of FIG. 8, the intermediate layer 21 canfurthermore, especially advantageously, be used simultaneously as anadhesion promoter and etch stop layer and as a shielding layer betweenthe waveguide layer 23 and the liquid traps 22.

The fundamental mode of operation of the sensor arrangement 1 describedabove will be explained below.

By application of an alternating voltage to the electrodes or transducerprongs 7 of one of the interdigital transducers 6 described above,alternating mechanical stresses are created in the substrate 5 becauseof the inverse piezoelectric effect, and these stresses result in anacoustic shear wave that runs perpendicular to the interdigitaltransducers 6 through the substrate 5.

If, when acoustic waveguide layers are used, for instance in a sensorarrangement for acoustic Love mode waves, the shear wave speed in thewaveguide layer 16, 23 is less than in the substrate 5, the result is aconcentration of the acoustical energy below and in this layer (theso-called waveguide effect). The resultant surface wave type is called aLove wave. These acoustic waveguide modes have an increased sensitivitycompared to the general shear modes, but the propagation damping of thewave is also affected by the waveguide layer 16, 23. If the propagationconditions of the acoustic wave change, then the propagation speed andthe damping are affected, so that a measurement of these wave parametersprovides information about the variables involved.

If a liquid measurement medium is located on a basic sensor elementhaving the interdigital transducers 6, the result is a viscous coupling;that is, a thin film of liquid on the surface of the basic sensorelement is forced to go along with the shear oscillations. The effectiveheight of the co-oscillating liquid film (decay length) is directlydependent on the viscosity and the frequency. The viscous couplingcauses a decrease in the propagation speed of the acoustic wave and anincrease in the wave damping in proportion to the root of thedensity-viscosity product.

If discontinuities acting as liquid traps 17 are now present on thesurface of one of the basic sensor elements, then the propagation speedis reduced by a second influence, which is dependent on the liquiddensity and on the liquid volume enclosed in the liquid traps 17. Thechange in propagation speed can be measured for instance if a basicsensor element, for instance with the delay line of FIG. 3 or thereflectors 14 of FIGS. 4 and 5, is used as the frequency-determiningmember in an oscillator circuit. The change in resonant frequency ofsuch an oscillator is a measure for the change in speed of the wave.

If a basic sensor element having the IDTs 6 is also available that isnot exposed to the liquid as a measurement variable, then by mixing twooscillator frequencies, such interference variables as temperaturefactors can be compensated for, and furthermore the low-frequency signalΔf is directly available as an output variable.

FIG. 10 shows a basic exemplary embodiment of a circuit layout forascertaining the density and viscosity of a measurement liquid, with twooscillator circuits 30 and 31. The oscillator frequency f₁ of a basicelement having the IDTs 6 and liquid traps 17 in the first oscillatorcircuit 30 is mixed in a mixer 32 with the oscillator frequency f₂ ofthe oscillator circuit 31 without liquid traps; the mixed frequency Δfat the output of a low-pass filter 33 located downstream is, in goodapproximation, a measure for the liquid density, since the viscousinfluence acting on both oscillator circuits 30 and 31 is compensatedfor, as are any further interference variables. The prerequisite here isthat the sensitivity to viscous coupling be identical in both basicsensor elements.

The viscosity of the measurement liquid can be ascertained with thecircuit arrangement described, using the ascertained density, from theshift in the frequency of the basic sensor element without liquid trapsin the oscillator circuit 31, compared to the known frequency when themeasurement arrangement is operated without a measurement liquid.Analogously, the change in damping can also be used as a measurementvariable.

A further, expanded exemplary embodiment of a circuit arrangement fordetecting the measurement variables required is shown in FIG. 11. Inthis circuit arrangement, there is in addition an oscillator circuit 34with a measurement path without any passage therethrough of themeasurement liquid, so that this measurement path acts as a referenceelement that is not wetted.

This arrangement is advantageous especially whenever the two sensorelements, wetted with the measurement liquid, for technological reasonslack identical sensitivity to viscous interactions. Also in thisarrangement, possible component drifting is made more homogeneous, toimprove long-term stability.

From the resonant frequency shifts Δf₁ and Δf₂, it is possible on thebasis of the layout of FIG. 11, and given a known sensitivity of thecomponents to changes in density and viscosity, to ascertain the liquiddensity and, using the thus-known liquid density from Δf₂, theviscosity, as described above. Alternatively, once again the change indamping can be used as a measurement variable.

A third embodiment, not shown in the drawing, has two basic sensorelements, both of which are provided with liquid traps, and one of whichis in contact with the measurement liquid while the other is in contactwith air. The mixed frequency, obtained here in an analogous way to theexemplary embodiments of FIGS. 9-11, is dependent on the density and theroot of the density-viscosity product. The damping difference is thusalso the measure for the root of the density-viscosity product, since aslight increase in mass as a result of the measurement liquid in theliquid traps results in only a negligible change in damping.

Thus a damping measurement is absolutely necessary, but it isadvantageous that only two completely identical basic sensor elementsare necessary, for the same sensitivity, drift and mechanical crosssensitivity.

What is claimed is:
 1. A sensor arrangement for ascertaining the densityand the viscosity of a liquid, having an arrangement comprising at leasttwo basic sensor elements, at least one of which can be wetted with theliquid, and having electro-acoustical transducers (6) in the basicsensor elements for generating and detecting surface acoustic waves withpredetermined wave modes, from whose propagation behavior along ameasurement path a measure for the density and the viscosity of theliquid can be ascertained, characterized in that liquid traps (17) forthe liquid, which extend in an applicable measurement path, are disposedin a region of at least one of the basic sensor elements, parallel tothe direction of propagation of a surface acoustic wave.
 2. The sensorarrangement of claim 1, characterized in that the evaluated surfaceacoustic waves are horizontally polarized acoustic shear waves of theLove mode type.
 3. The sensor arrangement of claim 1, characterized inthat evaluated surface acoustic waves are horizontally polarizedacoustic shear waves of the SSBW or leaky wave type.
 4. The sensorarrangement of one of claim 1, characterized in that theelectro-acoustical transducers are formed of interdigital transducers(6), disposed on a substrate (5), whose transducer prongs (7) areembodied such that requisite wave modes can be generated with a suitableoscillator frequency.
 5. The sensor arrangement of claim 1,characterized in that each basic sensor element is embodied as a delayline with two interdigital transducers (6 a, 6 b) and with a propagationor measurement path (13) between the transducers.
 6. The sensorarrangement of 1, characterized in that each basic sensor element isembodied as a two-gate resonator with two side-by-side interdigitaltransducers (6 a, 6 b) and respective reflectors (14) located on theoutside of the transducers.
 7. The sensor arrangement of claim 1,characterized in that each basic sensor element is embodied as aone-gate resonator with one interdigital transducer (6) and respectivereflectors (14) located on the outside of the transducers.
 8. The sensorarrangement of claim 1, characterized in that the liquid traps (17) areformed by trenches or etched features (22) in a suitably structurablelayer (20) above the electro-acoustical transducers (6), optionally onan intermediate layer (21).
 9. The sensor arrangement of claim 1,characterized in that the liquid traps (17) are formed by trenches oretched features (22) in a suitably structurable layer (20) above theelectro-acoustical transducers (6), optionally with an outer metalshielding (26).
 10. The sensor arrangement of claim 1, characterized inthat the liquid traps (17) are formed by trenches or etched features(22) in a suitably structurable layer (20) above the electro-acousticaltransducers (6), and that an intermediate layer (21) located below thetransducer and a further acoustic waveguide layer (23), located betweenthe intermediate layer (21) and the electro-acoustical transducers (6),is present.
 11. The sensor arrangement of claim 1, characterized in thatthe liquid traps (17) are formed by trenches (22) extending parallel tothe direction of propagation of an acoustic wave.
 12. The sensorarrangement of claim 1, characterized in that the liquid traps (17) areformed from an arrangement of pits of circular or polygonal crosssection or from a spongelike surface structure.
 13. A method forascertaining the density and viscosity of a liquid, having a sensorarrangement of claim 1, characterized in that with a first oscillatorcircuit (30), which has a sensor element with liquid traps (17, 22), afirst oscillator frequency f₁ is generated, with a second oscillatorcircuit (31), which has a sensor element without liquid traps, a secondoscillator frequency f₂ is generated; that from a mixed frequency Δf ofthe two oscillator frequencies f₁ and f₂, the density of the liquid isascertained, and that from a frequency shift of the second oscillatorfrequency f₂ of the oscillator circuit (31), the viscosity of the liquidis ascertained in a measurement without liquid compared a themeasurement with liquid.
 14. A method for ascertaining the density andviscosity of a liquid, having a sensor arrangement claim 1,characterized in that with a first oscillator circuit (30), which has asensor element with liquid traps (17, 22), a first oscillator frequencyf₁ is generated; that with a second oscillator circuit (31), which has asensor element without liquid traps, a second oscillator frequency f₂ isgenerated; that with a third oscillator circuit (34), which has a sensorelement without liquid traps and without the liquid to be measured, athird oscillator-frequency f₃ is generated; that from a mixed frequencyΔf₁ of the first and third oscillator frequencies f₁ and f₃, and a mixedfrequency Δf₂ of the second and third oscillator frequencies f₂ and f₃,the density of the liquid is ascertained, and that from the mixedfrequency Δf₂, the viscosity of the liquid is ascertained.
 15. A methodfor ascertaining the density and viscosity of a liquid, having a sensorarrangement of claim 1, characterized in that with a first oscillatorcircuit, which has a sensor element with liquid traps, a firstoscillator frequency f₁ is generated; that with a second oscillatorcircuit, which has a sensor element without liquid traps and without theliquid to be measured, a second oscillator frequency f₂ is generated;that from a mixed frequency Δf of the first and second oscillatorfrequencies f₁ and f₂, a measurement signal is ascertained as a functionof the density and the root of the density-viscosity product of theliquid; and that from damping difference of the two oscillatorfrequencies f₁ and f₂, a measure for the root of the density-viscosityproduct is ascertained.